Continuously-acting hydrostatic-mechanical power-shift transmissions--such as DE Patent 31 47 447, f.i.--are very suitable for motor vehicles since they have a wide range of continuous adjustment and good efficiency factors. The transmission's relatively wide range of adjustment allows to operate the internal combustion engine on a preferred characteristics line. Some such characteristics lines could be, f.i., the curve for lowest fuel consumption, a characteristics line for good acceleration response, or one that addresses constant RPMs.
The direction of the power flow in the hydrostatic gearing is reversed during a speed shift. The pump and motor functions become inverted in such reversals. If the adjustable displacement-type machine operated as a pump in the previous gear, then, in order to cover the leakage fuel flows, as well as to achieve synchronous RPMs when shifting into the new gear, the machine's displacement volume V must be set at a higher value than the theoretical value of the displacement value V.sub.theoretical, which would exist in a leakage fuel-free operation. However, if in the previous gear this displacement-type machine operated as a motor, then, in the new gear--where it now operates as a pump--its displacement volume V would not be sufficient.
When a gear shift is about to be effected, the first step is to insert the new gear at synchronous RPMs, load-free. The load for the new gear is provided by making the corresponding modification in the displacement volume V. The high pressure in the hydrostatic gearing returns to the feeder pressure. At this point, the hydrostatic gearing ceases to provide support. Each gear, the previous as well as the new one, transmits one half of the torque to the driven (power-take-off) shaft. By further modifying the displacement volume, the hydrostatic gearing takes up load once more, whereby the motor and pump functions become reversed. When the volume modification is carried out correctly, the new gear receives the entire torque, and the previous gear can be separated practically load-free. However, the leakage fuel flows, and thus the necessary volume corrections, depend on the magnitude/number/extent of the pressure, the RPMs, temperature, fuel viscosity, angle of traverse, manufacturing tolerances and wear. As a rule, different leakage fuel flows are present in each gear shift action. Therefore, it is necessary to provide a relatively high clutching power in order to always be able to disengage the previous gear. However, this entails an abrupt shift of power from the previous gear to the new one. This creates bothersome jolts during shifting.
Claim 9 of the disclosure document DE 38 15 780 A1 contains the description of a measure that is meant to eliminate such shifting-induced jolts as much as possible. To achieve this end, the volume of the adjustable displacement-type machine is corrected prior to shifting out of the gear currently in use, according to the equation shown below: EQU V.sub.new =2V.sub.theoretical =V.sub.previous
In this way, it is being tried to correctly take into consideration the leakage fuel flows that are present during each particular gear shift action.
However, the pressure as well as the RPMs--and, consequently, the leakage fuel flows--can still change during the shifting action proper. Furthermore, there exists the possibility that the vehicle goes from pull action to push action during shifting, with the result that the pump and motor functions of the displacement-type machines are changed around as well. Therefore, in such cases, the foregoing equation is not the solution.
When the provided shifting power is used during rapid shifting actions--f.i. during a "kick-down"--it can happen that the time span may not be sufficient to separate the toothed clutch load-free, and it will bear on (make contact with) the rear profile, creating bracing. The previous gear is then pulled out under these conditions and jolts occur.
Drawing 5 of DE 3815 780 A1 shows the diagram for the control and adjustment of a continuously-acting hydrostatic-mechanical power-shift transmission.
U.S. Pat. No. 3,302,475 and GB Patent 2166 206A are also relevant for the description of the present state of the technology with regard to the present invention.
Toothed clutches with straight frontal profiles and sloped rear profiles are used as override clutches, as per U.S. Pat. No. 3,302,475 f.i. On the one hand, the two clutch parts (halves) pertain to a stage in the low-speed group gear, and, on the other, to a stage in a high-speed group gear. If the latter (which idles in active low-speed group gear) is connected, hydraulically, to the drive shaft via a friction clutch, the hydraulically produced shifting power is eliminated in the override clutch (the wheels pertaining to the override clutch are mounted on a shaft that runs parallel to the drive shaft); thus, the clutch is separated by deflecting the clutch part with the sloped profiles. At this point, the low-speed group gear idles, while the high-speed group gear transmits power. The shifting is effected without interrupting the tractive force. However, the gearing lacks the infinitely variable adjustment feature. Furthermore, an interruption is required in order to shift the gears which are subordinated to the described group gears.
Synchronizing blocking devices (dogs) for toothed clutches are known, f.i. from GB Patent 2 166 206 A.